FAILURE & CONDITION
MONITORING
OF
INSTRUMENT TRANSFORMERS
18TH MAY 2021
PRESENTATION BY
G V Akre
Director-Technical
Hivoltrans Electricals Pvt Ltd.
Halol,Gujarat
Chairman
Society of Power Engineers(I)
Vadodara Chapter
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INTRODUCTION
Instrument transformers play vital role in power
system and their operational reliability is very
important.
There are frequent cases of catastrophic failure of
Instrument Transformers.
Explosive failure may cause damages to adjoining
equipments causing considerable loss of asset &
injuries (sometimes fatal) to the personnel.
Failure of ITs leads to malfunctioning of system
protection and costly power outage.
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INTRODUCTION
• Large population of equipment commissioned during
last 2 to 3 decades are ageing out which is cause of
concern as the failure of aged CTs is predominant
and non predictable.
• Also the operations of the grids and power stations
are subjected to higher loads,and higher voltage
surges nowdays and that can accelerate the aging
of old transformers.
• It is very important to understand failure
mechanism of transformers and how to avoid
disastrous explosions.
• . Status of ageing of transformer can beundestood
by
condition monitoring.
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STUDIES ON OIP INSULATION AGING
Studies are made by CIGRE and other organisations to
find solutions to aging of Oil Impregnated Paper(OIP)
insulation like,
Which diagnostic indicators are correlated with
ageing?
What are the ageing mechanisms? Which parameters
like temperature, water, dissolved by-products act on
insulation ageing?
How can the insulation ageing rate be slowed?
Which maintenance practices are recommended to
extend transformer life?
How ageing can be reduced by keeping oxygen and
water away from the insulation systems
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BATH TUB CURVE
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BATH TUB CURVE
• The curve is a general representation of failure
pattern of electrical equipments from the initial
years of commissioning to the end of operating life.
• Failure during first few months/years are usually due
to manufacturing defects, transit damages,
installation problems, leakages etc as explained in
detail ahead.
• Random failure mode includes failures due to service
condition, system switching surges, lightning impulses,
ineffective maintenance etc.The rate of failure
during this period is quite low.
• Wear out failures includes natural aging process
resulting into dielectric degradation. Failure rate
during this period is quite high’ sudden and
unpredictable.
• This period normally starts after two decades.
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CATASTROPHIC FAILURE
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Factors Causing failure
Following factors are mainly responsible for degradation
of insulation.
1. Manufacturing defects
– Poor Quality of Raw Material
– Residual Moisture-inadequate drying of insulation
Resulting into
• Over stressing- electrical stress
• Partial Discharges
• Dielectric loss –overheating of insulation
• Acceleration of aging process
– Poor quality control during manufacturing
– Poor hermetic sealing resulting into
• moisture ingression
• air ingression
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Factors causing failure
2.Service Condition
• Mis-handling, mechanical damages during
transit/installation
• Unattended leakages-moisture ingress
• Leakage of N2 gas used for harmetic sealing
• Leakages through aged out pressure release device
• Lightning and switching surges
• Ferro resonance
• Polluted atmosphere
• Poor maintenance program
• Terminal connectors-loosened contacts
• Old installations, frequent interruption / overcurrents
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Factors responsible for failure
3.Natural Ageing
 Stresses like dielectric, thermal,
mechanical and chemical are
continuously acting on paper and oil
insulation during its life.
 The stresses cause the natural ageing
process of degradation of insulation.
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DEGRADATION PROCESS OF INSULATION
• Insulation paper is made from cellulose
fiber.
• Paper is very hygroscopic by nature &
readily absorbs moisture from the
surrounding.
• Aging is influenced by degradation of
cellulose and oil due to different
stresses-Elecrical,Mechanical,Thermal
and Chemical.
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DEGRADATION PROCESS OF INSULATION
• Ageing accelerates in presence of oxygen
and moisture.
• Thermal degradation causes oxidation &
reduces strength of paper (degree of
polymerization-DP).
• Degradation process produces fault
gases,mainly H2, CO2, CO, Methane
(CH4), H2O, acids and sludge.
• Water formed acts as catalyst to
accelerate further degradation
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DEGRADATION PROCESS OF INSULATION
Effects of Degradation
– Oxygen formed during aging process
mainly affects the oil causing oxidation
– Affects resistivity & insulation properties
of oil & paper
– Increases tan delta, which in turn
increases dielectric heating.
– Weak insulation causes PDs of higher
values causing local heating and burning of
insulation.
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DEGRADATION PROCESS OF INSULATION
Effects of Degradation
• Tan delta of 0.5% produces 20 watts in 245
kV insulation & 100 watts in 420 kV CT
insulation as dielectric loss (heat loss)
• A poorly dried transformer with higher
moisture content and gas (particularly O2)
drastically reduces life expectancy.
• Initial moisture and O2 causes early
degradation reducing life expectancy.
Thus, it can be seen that presence of
moisture and O2 can cause speedy
degradation of insulation.
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MOISTURE TRANSITION
BETWEEN OIL & PAPER INSULATION
• Moisture present in CT is absorbed by paper and oil in a
proportion depending on temperature
• There is always equilibrium between moisture in cellulose
of paper and oil in an insulation system at any given
constant temperature.
• With change in temp, the equilibrium is disturbed and
transition of moisture takes place between oil & paper.
• At higher temperature, the water absorption capacity in oil
increases and that of paper reduces
• Thus,mosture migrates to oil from paper and oil insulation
gets more moisture.
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MOISTURE TRANSITION
• As temperature reduces moisture migrates
from oil to paper.
• In summer,during sunny days and higher
loads, temp increases considerablyand
migration of water takes place from paper
to oil.
• During cool nights the reverse takes place
& moisture migrates from oil to paper
• With high variations in the temperature
the concentration of the moisture in any of
insulation media may become critical and
break down may take place.
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Moisture equilibrium in OIP insulation
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MOISTURE TRANSITION
Illustration from the experimental graph.
• At 60°C with moisture of 20 ppm in oil, the paper contains
2.5% moisture by weight
• At 20°C with moisture of 20 ppm in oil, the paper can
contain 7% of moisture by weight.
• The paper with initial higher percentage of total moisture
present in CT may cause reduction of dielectric strength
significantly and leads to insulation breakdown.
• Hence if Initial Moisture is less or Equipment are well dried
in factory ,then migrssion of mosture is limited and can not
cause failure.
• Above experimental finding is also experienced by fact
that max failures are taking place during cool nights of
hot summers in India, when max variation in temp takes
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place.
MOISTURE TRANSITION
Thus, it is important to observe following:
• In new transformers, the paper and oil must be perfectly
dried to minimise the initial moisture content. (preferably 5
ppm in oil and max 0.5% in paper.) The moisture accelerate
the degradation process during initial years.
• ITs are minimum oil equipment and oil is not changed during
its lifetime, hence they must be hermetically sealed, with
metallic bellows or Nitrogen gas.
• Leakages must be attended immediately. Even minor leakage
can absorb substantial moisture and O2 from atmosphere in
course of time.
• With above precautions, the risk of absorption of external
moisture will be minimum.
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DIAGNOSTIC TESTS AND CONDITION MONITORING
• Condition monitoring may be defined as a process of
monitoring the characteristics during operation of
equipment and find out the changes and trends of the
status of the insulation system which can be used to predict
the need for maintenance before serious deterioration
occurs.
• Thus it determines the health of the equipment and routine
maintenance can be rescheduled as required. This reduces
the unnecessary cost of maintenance.
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Diagnostic tests
• Electrical Diagnostic Tests
– Test for Insulation Resistance
– Polarization Index
– Tan Delta and Capacitance Measurement
– Resistivity, tanδ and BDV of oil
– Partial Discharge Test on site
• Chemical Analysis
(A) -Test for Water content
-Dissolved Gas Analysis (DGA)
-IFT
(B) -Degree of Polymerization (DP)
-Furan Analysis
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Diagnostic tests (Cont'ed)
– Other Tests and Inspection
• Infrared Thermography
• Visual inspection for leaks, polluted insulators, corona
discharges.
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Insulation Resistance Test
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Initial screening test for equipments.
To use high range 1000,000 mΩ, 5 to 10 kV IR tester
Test during fair weather condition.
Frequency of test may be high for better results.
Gives initial warning to engineers to decide further
investigation.
• Normally useful for medium and low voltage trans upto72.5 kV.
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Polarization Index
Ratio of IR values at end of 600 seconds and 60 seconds.
Possible only with high range Meggar.
Value of 1.3 to 2.0 indicates good insulation for instrument
transformers.
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Tan δ and capacitance Test
• One of the best diagnostic test to monitor
insulation condition.
• Concept of tanδ can be explained by considering
insulation as capacitor.
• An ideal capacitor carries only capacitive current
(Ic) which leads the voltage by 90º.
• But actual capacitor formed by transformer
insulation conducts resistive current (Ir) due to
moisture and impurities. The resultant current
(I) is vectorial sum of Ic and Ir and leads
voltage by slightly less than 90º. The angle
between Ic and I is called loss angle and tangent
18thcalled
May 2021 “tan δ”.
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Tan δ and capacitance Test
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Tan δ and capacitance Test
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Tan-delta Test
Higher tanδ indicates degraded insulation.
• Comparison between periodic measurements
reveal trend in deterioration of insulation.If
trend is increasing the equipment should be
replaced.
• Increase in tan δ may be due to Moisture,
contaminated oil, internal PDs etc.
• Rise in tanδ with rise in voltage indicates high
moisture /deterioration and steeper rise
indicates major defects.
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Tan-delta Test
• Normal value for new CTs can be 0.18 % to
0.4% for new Instrument Transformers
• For Old equipment with more than 0.7%
should be closely monitored and if
increasing trend is observed the equipment
should be replaced.
• Higher tanδ produces higher dielectric loss
and if paper-oil insulation is not able
dissipate the heat it will increase
temperature leading to thermal
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Partial Discharge Test at Site
• More advanced site test—Off line / online.
• PDs are partial discharege in the insulation due to
Voids,Sharp edges of electrodes,damage in
insulation during transit,mechanical forces due to
surges during operation,deteriorated insulation
etc.
• Pds are very high frequency discharges of low
voltages that are picked up by special instruments.
• Direct online method receives signals from PF
terminal of CT.
• Signals are mixed with unwanted external
discharges.
• Special filters and microprocessor base
instruments for separating internal discharges
from external are used.
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• 2021
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Interpretation of results of Resistivity, Tan δ and
BDV ,IFT,Acid Number of oil
• Tan-delta increase indicates oxidation, contamination
and suspended water particles.
• Low BDV indicates moisture, contamination due to
oxidation.
• Low resistivity indicates presence of suspended
water, acidic oxidation etc.
• Decrease in Resistivity with increase in tan δ indicates
soluble contaminants and aging.
• Satisfactory resistivity at 90º compared with low
resistivity at ambient temperature indicates
moisture.
• IFT (Inter Facial Tension )detects contaminants and
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oxidation products inGVA-SPE-Parul
insulating oils.
Dissolved Gas Analysis (DGA)
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– Incipient faults cause PDs, Corona, & Arcing and
generates heat with very high range of temperatures in
affected section.
– Oil and cellulose insulation decomposes and produces
different gases at different temperatures.
– Significant gases due to oil decomposition are H2,
methane (CH4), ethane (C2H6), ethylene (C2H4)and
acetylen(C2H2).
– CO2, CO and O2 are produced as the result of
degradation of cellulose during aging process and due to
hotspot in insulation.
– By using the Curves and Software with inputs of Fault
Gases from DGA, type of Faults can be found out in the
insulation.
- DGA should be done if Tan-D tests indicates
deterioration of insulation.
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CHEMICAL ANALYSIS TESTS ( Furan, Moisture
Measurement)
• Water Content Test
– Water content in oil should be less than 10 /25
ppm, target to be 5 /15ppm (New /Old CTs).
– High content results into lower BDV and higher
tan δ and conductivity.
– Moisture in oil may provide some information
about moister content in paper insulation, but it
may not be always true. Temperature at which
samples taken is important due to Transition
Theory of moisture
– Above tests in addition to DGA and Furan can give
more reliable information about insulation
condition.
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Degree of Polymerization Measurement (IEC 450)
DP
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Determines thermal aging of solid cellulose insulation.
Quality of cellulose is measured in DP.
Measures indicate tensile strength of paper.
New Kraft paper has DP : 1000 to 1500.
With long service DP maybe: 200 to 250.
DP value of 150 to 200 indicates mechanical strength of
only 20% of initial strength and is considered to be the
end of insulation life.
• Most accurate test, but sample of paper must be
obtained which is impossible for IT.
• Results of DP Used to verify remaining life of the
equipment.
• May be performed to establish cause of failure in failed
equipments.
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Furan Analysis
• Furans form as degradation product of solid insulation
and are soluble in oil.
• Furan analysis is performed by drawing sample from oil
from operating transformer.
• Easy test as compared to DP test as transformer is not
required to be opened.
• Interpretation of results are not as reliable as DP.
• Furan analysis, DGA Test & DP Test combined offers
very reliable conclusion about deterioration of insulation.
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INFRARED THERMOGRAPHY
• Infrared thermal imaging system is a technique
involving infrared camera, software and computer
to measure temperature of equipment on line from
safe distance.
• Camera senses infrared radiation from heated
components of equipment in Switch yard. A
computer processes this information and displays
the images of components with different colors
depending upon the temperatures.
• By comparing the difference in similar parts in the
equally loaded CTs/CVTs/PTs in different phases,
abnormally heated component is pointed out.
• Further investigation & corrective action is taken
to avoid further damages.
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Infrared Thermal Image
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Infrared Thermal Image
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Case studies.
• Failure due to PDs
• Bellow Expansion used for hermetic
sealing.
• Diagnosis after Thermography
• Failure due to Ferro-resonance
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CATASTROPHIC FAILURE(Case study)
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Expansion of Bellows due to PDs
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Infrared Thermal Image (Case Study)
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Partial Discharge Failures (Case Studies)
• 220 CTs were transported to MP by truck
crossing hilly roads,cased internal loosening
and knocking of insulated primary winding
with metal flange.This caused small
depression of the neck portion of hv
insulation.This depression caused PDs and
generation of fault gases which expanded the
bellow used for harmetic sealing.The CTs
were brought back to works and tested which
confirmed high levels of PDs due to dent in
the neck insulation.Tan-D,DGA also indicated
deterioration of insulation.
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VT Failures due to Ferro-Resonance(case study)
• 2 nos new132 VTs in operation at Durgapur Steel Plants
failed one after other within a week of commissioning.
• Site visit revealed that the area was populated with many
small steel plants working on Arc Furnaces.Many companies
were not using the capacitor banks also which are must for
arc furnaces.
• The supply system was facing continuous surges which
caused ferro-resonance in VTs developing high voltage and
drew high currents in primary winding due to saturation of
core.High current for few minutes overheated primary
leading to insultion burnout and then explosion.
• All VT secondaries were connected to special ferroresonance damping devices solving the problem.
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Ferro-resonance of 66 VTs with Connecting HV
Cables (Case Study)
• 66 VTs connected with HV Cables from 132
kV substation at a distance.The cables forms
high capacitance that was acting in parallel to
VT primary Windings.The VT primary acts as
the saturable reactor when subjected to HV
surges in the system
• Short circuit fault occurred at 132 kV line
,that caused severe surge in 66 kV side
connected with HV Cables.
• The surge saturated the core and varied
inductive reactance of Primary winding locked
in resonance with cable capacitance blasting 3
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nos of VTs simultenously.
MAINTENANCE PROGRAM
A general program for condition monitoring by users
1.
2.
3.
4.
Regular measurement of IR, PI, Tanδ-capacitance be
made at suitable intervals in fair dry weather and
recorded.
The variations from each periodical test be recorded
in a computerized database and the trend of
deterioration determined.
If the trend is indicative of progressive
deterioration, then oil sample may be drawn for
Moisture content, IFT,Acid numbers, DGA and
Furan test.
If oil tests confirm the deterioration, the instrument
transformer should be removed from services as
early as possible to avoid catastrophic failure.
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ONLINE CONDITION MONITORING
 As a result of compelling need felt by many utilities, online
monitoring systems have been developed for transformers
in more advanced countries to avoid unexpected failures.
On-line monitoring of critical network assets provides
information previously unavailable. This in turn allows better
asset management. It is fast maturing into a serious and
reliable network tool.
 Normally dissipation factor and partial discharge tests are
conducted on line using specially developed instruments.
Technique of on-line monitoring of DGA and evolved gases is
also developed, however it is mostly used for very costly
equipments like power transformers presently.
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ONLINE CONDITION MONITORING
The key advantages are:
1.
Relevant data collected and made visible on
network.
2.
Major help in delaying routine maintenance as long
as possible hence driving down costs.
3.
Costly and not easy to replace equipments are prime
candidates for on-line monitoring.
3.
Damage to the asset is minimized.
5.
Equipment need not be taken out of service
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CONCLUSIONS
• The degradation and subsequent failure of transformers
is the result of aging of cellulose from paper insulation at
elevated temperatures.
• Use of proper material and best manufacturing practice
is the first step to minimize the degradation of
insulation.
• However, for the instrument transformer to live its full
life of 20-25 years, it is important to adopt efficient
maintenance management by monitoring the condition of
insulation at site by the user.
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CONCLUSIONS (Cont'ed)
• In our country, very few utilities may be
using some or all of the diagnostic techniques.
• Therefore, increased awareness and adoption
of these essential techniques is highly
desirable
• Effective condition-based maintenance
practices for substation plant assets will
result in reduced controllable operating costs
and improved utility performance.
• Proper investigations and analysis of failure
of the equipments, carried out worldwide has
resulted into improvement in electrical
equipment design.
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THANK YOU
G V Akre
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